Chlorite (i.e. sodium chlorite) is an oxidation agent, and its most important use is as a bleaching agent, preferably for textiles. Chlorite is also used for the local preparation of small amounts of chlorine dioxide by oxidation with chlorine. Such a product flow of chlorine dioxide is only contaminated by chloride ion and can therefore be used without separation stages for e.g. water purifying.
In spite of trials in other directions almost all chlorite is at present prepared by the chemical reduction of chlorine dioxide. If chlorine dioxide is led into sodium hydroxide, disproportioning takes place and chlorite and chlorate (i.e. sodium chlorate) are formed according to the following formula EQU 2 ClO.sub.2 + 2 NaOH = NaClO.sub.2 + NaClO.sub.3 + H.sub.2 O
thus, one mole of chlorite is obtained from two moles of chlorine dioxide. By adding a reducing agent it is theoretically possible to obtain an equimolar amount of chlorite from chlorine dioxide. A plurality of substances have been suggested as reducing agents, such as zinc, hydrogen peroxide, carbon powder, lead(II)oxide and sodium amalgam: EQU 2 ClO.sub.2 + Zn + 2 NaOH = 2 NaClO.sub.2 + Zn(OH).sub.2 I EQU 2 clO.sub.2 + H.sub.2 O.sub.2 + 2 NaOH = 2 NaClO.sub.2 + 0.sub.2 + 2 H.sub.2 O II EQU 4 clO.sub.2 + C + 6 OH.sup.- = 4 ClO.sub.2.sup.- + CO.sub.3.sup.2- + 3 H.sub.2 O III EQU 2 clO.sub.2 + PbO + 2 NaOH = 2 NaClO.sub.2 + PbO.sub.2 + H.sub.2 OIV EQU clO.sub.2 + Na.sub.(Hg) = NaClO.sub.2 (see British patent 764,019).V
when zinc is used as the reducing agent, zinc hydroxide or zinc carbonate is obtained as a by-product, which must be separated and worked up or deposited.
Hydrogen peroxide is an expensive chemical agent and does not provide any by-product of value at the reduction. Moreover, handling of hydrogen peroxide involves some safety problems. Since oxygen gas is the only by-product obtained, the separation problems are reduced in the working up.
When reducing with carbon an awkward carbon suspension must be handled, and carbonate is obtained as a by-product which must be separated.
The use of lead oxide in reduction will give the technically important chemical PbO.sub.2 as a by-product after separation. However, the use of lead in the process creates handling problems due to the toxicity of the substance.
Processes based on sodium amalgam as reducing agent have two disadvantages. Mercury is per se an inconvenient and toxic chemical. Moreover, the sodium content in the amalgam is low, and therefore large amounts of amalgam must be handled.
In all these processes an addition of reducing agent is required and the price of the added chemical decides the economy of the process to a high degree.
Thus, reduction by addition of chemical reducing agents involves a number of disadvantages. Attempts have also been made to prepare chlorite electrolytically. A direct electrolytic oxidation of chloride ion, chlorine or hypochlorite ion to chlorite ion does not seem to be thermodynamically possible, nor direct electrolytic reduction of chlorate to chlorite. British Pat. No. 644,309 discloses a process in which chlorine dioxide is electrolytically reduced to chlorite. In that process the chlorine dioxide is led to the cathode compartment, which is defined from the anode compartment by means of a diaphragm. However, this process has a number of drawbacks. In all of the embodiments according to the above British patent chemicals are required to make possible an anode process as a compliment to the chlorine oxide reduction at the cathode. Either sodium hydroxide, sodium chloride or sodium amalgam is added. Because of these necessary chemical additives the economy of the process is deteriorated, especially since by-products of a great value are not obtained either. As mentioned above, the use of amalgam brings other disadvantages in addition to the price factor. In the case of sodium chloride solution as anolyte it is also especially unfavourable that the cell halves are only separated by a diaphragm, as this means that the catholyte will be mixed with chloride ions, which catalyze a decomposition of chlorite formed.
In addition to the problems mentioned above in the preparation of chlorite by known methods there are additional problems associated with the preparation of the chlorine dioxide used as the starting material for the preparation of chlorite.
As stated above almost all chlorite preparation presently takes place by the reduction of chlorine dioxide. Since this chemical is extremely reactive, explosive and dangerous to the health, the transport thereof involves technical problems that are difficult to overcome. This means that the required chlorine dioxide must be prepared at the place of its reduction to chlorite. The greatest problems associated with the preparation of chlorine dioxide are connected with the working up of the residual solution from the reactor.
Normally chlorine dioxide is prepared by reduction of chlorate, and the most common processes for this can be summarized in the following gross formulas.: EQU NaClO.sub.3 + SO.sub.2 .fwdarw. 2 ClO.sub.2 + Na.sub.2 So.sub.4 EQU (the Mathieson process) VI EQU 2 naClO.sub.3 + CH.sub.3 OH + H.sub.2 SO.sub.4 .fwdarw. 2 ClO.sub.2 + HCOOH + H.sub.2 O + Na.sub.2 SO.sub.4
(the Solvay process) VII EQU naClO.sub.3 + NaCl + H.sub.2 SO.sub.4 .fwdarw.ClO.sub.2 + 1/2 Cl.sub.2 + H.sub.2 O + Na.sub.2 SO.sub.4 EQU (the Rapson R-2-process, see the Canadian patent 543,589) VIII
thus, the reducing agent in these processes is sulphur dioxide, methanol and chloride ion respectively. Other reducing agents, such as chromic acid or nitrogen oxides, have also been tried, but principally due to their higher price they have not been commercially utilized to a considerable degree.
All these processes take place with an excess of a strong acid, usually sulphuric acid, and therefore the spent liquor of the reactor will consist of sodium sulphate in strong sulphuric acid.
It is essential from an economical as well as environmental point of view that this liquor can be taken charge of and utilized. Previously it has happened that this liquor has quite simply been disposed of in the seqage system. However, more and more rigorous environmental demands have necessitated great efforts to take care of the spent liquor in another way.
By using a combined reactor/evaporator the sulhuric acid can be retained in the reactor and only solid sodium sulphate be withdrawn i.e. the least possible amount of by-product for this process (the Rapson R-3-process, see the Swedish Pat. No. 312,789).
By replacing the addition of sodium chloride and part of the addition of sulphuric acid with hydrochloric acid, which, however, is a more expensive chemical substance than sulphuric acid, the produced amount of sodium sulphate can be additionally reduced.
However, these processes still produce the difficultly usable sodium sulphate, and therefore it has been suggested to convert this into sodium chloride and sulphuric acid: EQU Na.sub.2 SO.sub.4 + 2 HCl.fwdarw.2 NaCl + H.sub.2 SO.sub.4 EQU (the Rapson R-4-process) IX
thus this process requires another reactor system after the chlorine dioxide reactor to recover sulphuric acid, and then the sodium chloride product is still not made useful.
A similar result, i.e. a sodium chloride containing spent liquor, is obtained if the chlorate reduction is carried out merely with hydrochloric acid: EQU NaClO.sub.3 + 2 HCl.fwdarw.ClO.sub.2 + 1/2 Cl.sub.2 + H.sub.2 O + NaClX
if in this process like in the previous ones one tries to achieve a high degree of conversion of the chlorate by means of a high acid content, some problems with the following side reaction will occur: EQU NaClO.sub.3 + 6 HCl.fwdarw.3 Cl.sub.2 + NaCl + 3 H.sub.2 O EQU (see the Canadian patent specification 920,773) XI
which will increase with increasing concentration of chloride ion, and partly with the spent liquor, which due to the high price of hydrochloric acid must not go to waste or due to its acidity cannot be economically worked up to chlorate in an electrolytic cell, either, as the solution must first be electrolytically neutralized.
Therefore processes of this kind (see the Swedish Pat. No. 155,759 and 337,007) operate with a low acid content, which permits electrolytic working up of the liquor to chlorate. In return these processes require a long residence time for the reaction between chlorate and hydrochloric acid and, consequently, several and big reactors. A high emperature is also used to increase the conversion rate, which brings increased risks of explosion, the mastering of which requires an increased number of apparatuses and process technical compromises.
Thus, to sum up, both purely chemical working up trials by the addition of reagents as well as working up trials by electrolysis of residual solutions from chlorine dioxide reactors have so far caused problems difficult to solve. Problems have also arisen both in acidification with sulphuric acid and with hydrochloric acid, which acids have so far been predominant.